This invention pertains to the field of spectroscopy and more specifically to nonlinear spectroscopy.
Nonlinear spectroscopy with laser sources can be performed with high resolution and high sensitivity. Nonlinear spectroscopic techniques take advantage of the nonlinearity of the medium under study. The nonlinearity is greatly enhanced at natural resonances and this property is used to detect resonances. This involves subjecting the medium to several input beams and measuring the intensity of the output beams as a function of the frequencies of the input beams. The sensitivity of nonlinear spectroscopy is often limited by background light due to fluorescence and scattering. One scheme to reduce the effects of background is described in an article entitled "Saturation Spectroscopy in Molecular Iodine by Intermodulated Fluorescence" by M. S. Sorem and A. L. Schawlow, Optics Communications, Vol. 5, No. 3, June 1972, pp. 148-151 involves synchronous phase-sensitive detection and measurement of the intensity of the output when the input beams are chopped at two different frequencies. The phase-sensitive detection must be made at the sum frequency. If one detects at the fundamental modulation frequencies a large background signal is observed because the scattered light and fluorescence from the sample will be modulated at those frequencies.